I really do understand and believe that if you love what you do, if you have a passion for what you do, you will have a much better chance of success, and the quality of your work will reflect your effort and interest.
How many of us, though, have the luxury of only working on what we have a passion for? For the most part, people do what they're doing in life to eek out a living. Most people don't "look forward" to Monday mornings, and instead focus on when the next long weekend is coming up.
At this point in my life, I can say that I LOVE putting in the countless hours building, developing and nurturing web sites. I LOVE soaking in as much info as I can, 'cause I really LOVE it.
But it's unrealistic to suggest that you should only build sites about things that you have a passion for. It helps, but it's certainly not essential. Whether my next site was about credit cards, weight loss, or male pattern baldness, I would put in whatever effort was necessary to make it work. I would enjoy the challenge, the work, and hopefully the profits. But the subject of the site doesn't necessarily have to be a burning desire for me.